Grain-oriented electrical steel sheet is obtained by hot rolling a silicon steel slab containing Si at for example 2 to 4%, annealing it, then cold rolling it one time or two times or more with process annealing in between to obtain a final sheet thickness, then is made into a final product by decarburization annealing it, then applying an annealing separating agent mainly comprised of MgO, performing finishing annealing to cause secondary recrystallization having a Goss orientation, further removing S, N, and other impurities, forming a glass film, then applying an insulating film agent and baking and heat flattening the sheet.
The grain-oriented electrical steel sheet obtained in this way is mainly used in electrical equipment, transformers, and the like as a core material and is required to have a high magnetic flux density and superior core loss. When grain-oriented electrical steel sheet is used as a transformer core, the grain-oriented electrical steel sheet coil is slit, is cut to predetermined lengths while being continuously unwound, and is stacked or wound by a core processing machine to obtain a stacked core or wound core. In the case of a wound core, compression forming, stress relief annealing, and winding work called “lacing” are performed to make the transformer. It is important in this transformer production process that the cutting, winding, and forming work can be performed easily. In particular, it is also important in wound core production that the adhesion of the insulating film be superior at the time of cutting and winding and the work environment not be impaired by dust production etc. and that the windability and annealing resistance be superior and the film performance, magnetic properties, and workability not be impaired.
The surface film of grain-oriented electrical steel sheet is usually comprised of forsterite film formed in the final finishing annealing and usually called a “glass film” and an insulating film processed over it. As technology for forming this insulating film, a tensile film comprised of colloidal silica, a phosphate, and a chromium compound was invented, disclosed, and industrialized by the present inventors in Japanese Patent Publication (B2) No. 53-28375. Further, a treatment agent comprised of a primary phosphate plus a fine grain colloidal silica of a grain size of 8 nm or less and a chromium compound is disclosed as shown in Japanese Patent Publication (A) No. 61-41778. Furthermore, Japanese Patent Publication (A) No. 3-39484 shows the technology of mixing colloidal silica of a grain size of 20 nm or less and colloidal silica of a grain size of 80 to 2000 nm with primary phosphates of Al, Mg, Ca, and Zn and a chromium compound to obtain a uniform protrusion effect at the insulating film surface and realize an improvement of winding (sliding property), annealing resistance, and film tension in the wound core fabrication process. Due to these, a tension effect and effect of improvement of the core processability can be attained and grain-oriented electrical steel sheet having superior magnetic properties and magnetostriction properties can be obtained.
These insulating films have all had chromium compounds added to and mixed with them considering the hygroscopicity after the film baking by a phosphate and the film seizure at the time of stress relief annealing.
The function of the chromium compound in the insulating film is to bring about the effects of improving the stickiness of the film and seizure and film tension during annealing and the like along with the effect of filling the porous film structure in a phosphate or a phosphate- and colloidal silica-based film and the effect of fixing the free phosphoric acid, which has hygroscopicity and degradability, remaining in the film component and forming a stable phosphoric acid-chromium compound after baking the insulating film. When the treatment solution uses chromic anhydride, a chromate, or a bichromate and contains hexavalent chromium, there are problems in the work environment during the coating work and in the work of treatment of the waste liquor. Further, in the film after the baking, while the Cr is reduced to trivalent chromium, the work environment is liable to be contaminated when dust is produced in the core fabrication process. As a countermeasure to this, research has been conducted on an insulating film agent not containing any chromium compound. Further, Japanese Patent Publication (B2) No. 57-9631 proposes a method of forming an insulating film comprising baking a treatment solution containing 20 parts by mass of colloidal silica as SiO2, 10 to 120 parts by mass of Al phosphate, 2 to 10 parts by mass of boric acid, and a total of 4 to 40 parts by mass of one or more ingredients selected from sulfates of Mg, Al, Fe, Co, Ni, and Zn at 300° C. or more.
Furthermore, Japanese Patent Publication (A) No. 7-180064 discloses a treatment agent comprising a solid solution type composite hydroxide composition of an average grain size of 1 μm or less represented by the general formula M2+1−xM3+1−x(OH)−2+x−nyAn−y.mH2O. Further, Japanese Patent Publication (A) No. 2000-178760 proposes a surface treatment agent for grain-oriented electrical steel sheet characterized by adding as an organic acid salt selected from Ca, Mn, Fe, Mg, Zn, Co, Ni, Cu, B, and Al one or more organic acid salts selected from formates, acetates, oxalates, tartarates, lactates, citrates, succinates, and salicylates.
These are all technologies able to exhibit a film tension effect and exhibit their effects accordingly. However, in the case of the technology proposed in Japanese Patent Publication (B2) No. 57-9631, there are problems of discoloration, insulation, corrosion resistance, and the like during annealing by the sulfuric acid ions of the sulfates added. Further, the technology proposed in Japanese Patent Publication (A) No. 2000-178760 may have the problem of color tone due to the organic substance for dissolving the metal elements and the problem of solution stability. If compared with the conventional chromium-containing insulating film agent in this way, overall it is difficult to say that the film performance has been sufficiently improved. Further improvement has been desired.